1. Field of the Invention
The present invention relates to an operational amplifier (op amp), and more particularly, to an operation amplifier circuit having a digital driving circuitry and an analog driving circuitry.
2. Description of the Prior Art
The operational amplifier (op amp) is an essential circuit building block of universal importance. A reason for the popularity of the op amp is its versatility; a great amount of operations can be done with op amps. Normally, an operational amplifier circuit is used to drive a load according to an analog input signal. However, if a digital input signal is adopted, a digital-to-analog converter (DAC) must be set in the signal input end of the operational amplifier circuit to convert the digital input signal into an analog input signal. The operational amplifier circuit can then drive the load according to the analog input signal.
For instance, the liquid crystal display (LCD) source driver is a kind of device that needs to utilize an operational amplifier circuit including a DAC to drive a LCD display unit according to a digital input signal. FIG. 1 shows a schematic diagram of a conventional operational amplifier circuit 100 utilized in a LCD source driver. The LCD display unit driven by the operational amplifier circuit 100 is represented by a block “load 190” in FIG. 1. The operational amplifier circuit 100 shown in FIG. 1 includes a DAC 110 and an op amp 120. The DAC 110 converts a digital input signal SIN into an analog input signal S′IN. The op amp 120 then drives the load 190 according to the analog input signal S′IN.
The op amp 120 can be thought of as an analog driving circuitry of the conventional operational amplifier circuit 100, and is fully responsible for driving the load 190. To make sure that the op amp 120 is qualified for driving the load 190 swiftly, sizes of circuit components (such as transistors) of the op amp 120 should be large enough, and bias currents utilized by the op amp 120 cannot be too small. Therefore, some integrated circuit (IC) area is wasted, and some power is consumed unnecessarily.